The most common of all arrhythmias, atrial fibrillation, is associated with approximately 150,000 strokes per year in the United States alone and yet there is presently no medical or surgical therapy capable of consistently curing this arrhythmia. In addition, the most common non- pharmacologic therapy for refractory ischemic ventricular tachycardia and/or fibrillation (the AICD) is designed not to prevent or cure the arrhythmia, but only to terminate it quickly enough to prevent sudden death. It is our belief that development of curative surgical procedures for these complex arrhythmias depends upon pursuing a methodical, scientific approach to the identification of the anatomic- electrophysiologic substrates responsible for their genesis which will lead to a clearer understanding of their basic underlying mechanisms. Thus, the objective of this competitive grant renewal is to continue our efforts towards the development of safe and effective surgical procedures for the treatment of complex supraventricular and ventricular tachyarrhythmias. This objective includes not only the design and refinement of new surgical techniques, but also the development of a more rapid and accurate electrophysiologic mapping system to guide these new surgical procedures. This grant proposal is divided into four sections: SECTION I is directed towards developing the most efficacious surgical procedure for the treatment of atrial flutter and fibrillation. Atrial fibrillation is currently being cured in otherwise healthy patients by the Maze Procedure (15 patients with 2-month to 3-year follow-up), but this procedure is too time-consuming to perform in the elderly or seriously ill, or as an adjunct to mitral valve surgery. The Maze Procedure also blunts the postoperative maximum sinus rate response to exercise. Our proposed modifications shorten the procedure and lessen the likelihood of postoperative sinus node dysfunction. In addition, canine models of standard atrial flutter and of the "slow atrial flutter" that follows the Mustard Procedure will be employed to develop surgical procedures to treat these recalcitrant arrhythmias. The objective of Section II is to develop a closed-chest cryosurgical procedure, based on the current open-heart technique, for the treatment of A-V node reentry tachycardia by passing a specially-designed cryoprobe into the right atrium via the internal jugular vein. The last two sections describe the development of a new computerized multipoint intraoperative mapping system capable of providing on-line, 3-dimensional static and dynamic (movie) activation time maps of atrial tachyarrhythmias (SECTION III) and potential distribution maps of ventricular tachyarrhythmias (SECTION IV). The proposed projects should result in safer and more effective surgical procedures for the treatment of the most common and hazardous atrial and ventricular tachyarrhythmias.